The present invention relates to an optical ADD-DROP module which can be expanded modularly.
When possible, wavelength-division multiplex systems (WDM systems) and dense wavelength-division multiplex systems (DWDM systems) are implemented as purely optical systems for cost reasons. In these systems, in order to extract individual channels, or more precisely for extracting signals of a particular wavelength, and to add channels (signals of the same wavelength) it is necessary to provide ADD-DROP multiplexers. To allow flexibility in responding to customer requirements, it must be possible to configure the injection and extraction of wavelengths remotely. It is also desirable that such operations are performed with minimum intervention in the system.
Until now, static ADD-DROP modules have been developed and are preferred. Marconi produces an ADD-DROP multiplexer in which a supplied WDM signal is split into parallel channels which are either switched through or are extracted and injected. The channels are combined again at the output of the ADD-DROP multiplexer via a WDM multiplexer. A drawback to this concept, is that with this arrangement it is not possible to expand the ADD-DROP capacity.
It is also known to connect a number of ADD-DROP modules in series. In this case, however, uninterruptable expansion is not possible. A serious disadvantage of this arrangement, however, is the high insertion loss.
It is the object of the present invention to specify an expandable ADD-DROP module with low insertion loss.
This object is accomplished by an optical ADD-DROP module according to the present invention. In an embodiment of the invention, the optical ADD-DROP module is modularly expandable such that additional ADD-DROP modules may be cascaded as necessary to meet requirements. An ADD-DROP module according to the invention includes a first filter arrangement. A WDM signal is supplied to the first filter which selects at least one droppable channel from the input WDM signal and passes the remaining channels to a second filter device. The ADD-DROP module includes at least one ADD-DROP switching device. The droppable channel selected by the first filter arrangement is supplied to at least one ADD-DROP switching device, and the ADD-DROP switching device has changeover switches for either switching the selected droppable channel through, or for dropping and adding a corresponding channel.
Meanwhile, channels to be looped through from the first filter arrangement are supplied to a cascading switching unit. The cascade switching unit either switches the channels through or supplies them to cascading terminals which may be connected to a second ADD-DROP module essentially identical to the first. When an additional ADD-DROP module is connected to the cascading terminals, the cascade switching unit may supply the additional looped through channels to second the ADD-DROP module and receive the WDM signal output therefrom. An ADD filter arrangement is provided in the first ADD-DROP module for combining the droppable channels and the channels to be looped through from the first ADD-DROP module, as well as the WDM signal output from the second cascaded ADD-DROP module if such a second module is necessary. The combined droppable channels and the channels looped through the first ADD-DROP module and the WDM signal output from second ADD-DROP module form a newly configured WDM output signal.
The special advantage of the ADD-DROP module of the present invention lies in the fact that the signals xe2x80x9cto be looped throughxe2x80x9d which are not supplied to ADD-DROP switching units, are only slightly attenuated. They can be supplied to another ADD-DROP module of at least approximately the same configuration via the cascading switching unit.
An ADD-DROP module with drop capabilities for two channels is particularly advantageous. This ADD-DROP capacity is usually adequate in a great number of cases. However, in those cases when two channel capacity is insufficient, expansion with another module as provided by the ADD-DROP module of the present invention results in an ADD-DROP capacity of four channels which is always sufficient for almost all applications.
A wavelength-division demultiplexer which has passbands of different widths can be used as the filter unit for selecting the ADD-DROP channels. This demultiplexer can also be advantageously constructed of individual optical filters, preferably as a Bragg grating. In these filters, the reflected waveband is attenuated only slightly and can, therefore, be supplied to the cascaded ADD-DROP module.
Further, the optical switching units used are preferably 2:2 switches in which the signals from two inputs can be arbitrarily switched through to two outputs.
To obtain similar signal levels in all channels, it is appropriate to provide optical attenuating sections in the signal paths. This applies particularly to the signal path of the channels to be looped through, which cannot be dropped. It is particularly advantageous in this arrangement to insert a constant-value optical attenuation section into the signal path of the looped through channels and to connect variable attenuation sections in series with the ADD-DROP switching units. Also the selection of the dropped channels can be improved further by an additional filter.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.